For a long time, saponified (solution-type) rosin sizing agents have been used in the so-called acidic paper-making process as a paper sizing agent for internal addition such sizing agents being used together with aluminum sulfate. However, at a low addition level, these sizing agents can achieve only a poor sizing effect. Furthermore, in systems having an elevated water temperature or an almost neutral pH range (such as in the expanded use of closed drainage systems) it is known that the sizing effect is deteriorated. In order to overcome these inherent disadvantages of the saponified rosin sizing agents, emulsion sizing agents have been developed. However, at the present time, these emulsion sizing agents are not always satisfactory, since, at a low addition level, they can achieve only a limited sizing effect.
In recent years, sizing agents prepared by alkali-saponifying alkenylsuccinic acids have been employed as a sizing agent which give a good-sizing effect even at a low addition ratio and, thus, compensates for the disadvantages of the above-mentioned rosin sizing agents (e.g., see JP-A-58-214598, wherein the term "JP-A", as used herein, means an "unexamined published Japanese patent application"). However, at a high temperature or in an almost neutral pH range, these sizing agents are disadvantageous in that they suffer from deterioration in the sizing effect in paper-making similar to the saponified rosin sizing agents.
Furthermore, U.S. Pat. No. 3,821,069 discloses a process wherein a mixture of an alkenylsuccinic anhydride blended with an emulsifier is emulsified in a solution of cationized starch or water at a low concentration of from about 0.5 to 3%, and the alkenylsuccinic acid-series emulsion sizing agent thus obtained is used as a sizing agent for neutral paper-making.
The function mechanism of an alkenylsuccinic anhydride in the neutral paper-making process proceeds as follows. The anhydride group of the alkenylsuccinic anhydride is fixed on pulp fibers through a direct reaction with the hydroxyl group of pulp. Thus, the sizing effect is exerted. In the neutral paper-making process, it is, therefore, necessary to add the conventional alkenylsuccinic anhydride as such (i.e., being in the form of an anhydride) to a pulp slurry. However, the alkenylsuccinic anhydride is highly reactive with water too. When the alkenylsuccinic anhydride is preliminarily emulsified and dispersed in water, it would quickly undergo a reaction with water and thus lose its anhydride group. As a result, the alkenylsuccinic anhydride loses its function as a neutral sizing agent. Furthermore, the emulsified state is modified during the process of the conversion of the alkenylsuccinic anhydride into the corresponding alkenylsuccinic acid, thus causing aggregation, precipitation and separation. That is to say, when an emulsion sizing agent comprising an alkenylsuccinic anhydride is used in the form of an aqueous dispersion in a neutral paper-making process, it can be stored for only several hours. It is, therefore, impossible to provide such a sizing agent in the form of a preliminarily emulsified product having a high concentration. Thus, it should be emulsified and dispersed with the use of an emulsifying machine immediately before the paper-making. In addition to this problem in handling, the emulsion sizing agent comprising an alkenylsuccinic anhydride is disadvantageous in that it requires a long rise time for exerting the sizing effect in an acidic region where aluminum sulfate is used as an adhesion promoter and exerts only a poor sizing effect immediately after the completion of the paper-making.
As discussed above, a conventional alkenylsuccinic anhydride emulsion exerts only a poor sizing effect immediately after paper-making in the acidic paper-making process. This is seemingly because this emulsion sizing agent is fixed on pulp fibers as such (i.e., in the form of the acid anhydride) and, therefore, reacts slowly with the pulp in an acidic region. Furthermore, a long time is required for the conversion of the alkenylsuccinic anhydride into the corresponding alkenylsuccinic acid through reaction with water and the subsequent reaction with aluminum sulfate, thus achieving the desired sizing effect. Provided that the alkenylsuccinic anhydride can be emulsified into the alkenylsuccinic acid, the reaction with aluminum sulfate would rapidly proceed and, as a result, an excellent sizing effect might be achieved even in the acidic region.
However, an alkenylsuccinic acid is a highly hydrophilic substance and, therefore, is difficult to emulsify. Thus, it is difficult by conventional techniques to give an emulsion thereof which has a high concentration and can be sustained in a stable state for a long time. When an alkenylsuccinic anhydride is emulsified by a conventional method such as described above, the alkenylsuccinic anhydride reacts with water in the emulsion and changes into the alkenylsuccinic acid, thus giving an emulsion of the alkenylsuccinic acid. During the process of the conversion of the alkenylsuccinic anhydride into the alkenylsuccinic acid, the emulsified state is modified, which makes it impossible to obtain a stable emulsion of the alkenylsuccinic acid. That is to say, even though an emulsion containing alkenylsuccinic anhydride having a high concentration might be temporarily prepared, said alkenylsuccinic anhydride would react with water in the emulsion and change into the alkenylsuccinic acid while simultaneously causing aggregation, precipitation and separation and, therefore, no stable emulsion can be sustained. Thus, it has been thus far impossible to give a highly stable emulsion containing an alkenylsuccinic acid in a high concentration.